So, If you you make a meal for your sims at the high school... they leave their plates in the hallway and refuse to clean it up.

Because there is no sink or trashcan to dump the plates.

There are! Upstairs in the washrooms and even in the home ec class...
I now have plate all over a hallway after a cooking demo

I never visited downtown high after finishing the quest. So I never did cooking hobby there. Just checked, the sink in home ec class gives no interactive options so I think it's just a deco. The sink on the floor above should work for cleaning plates though.
Also how do you cook a meal there? All I'm getting is class clown hobby options..

So, If you you make a meal for your sims at the high school... they leave their plates in the hallway and refuse to clean it up.

If you have stacks of plates, you can ask a sim to 'grab a plate' then CANCEL the action once he takes a plate from stack. You can do this to remove 1 by 1. Or you can call many sims and do the same. But don't let them finish the meal or you'll be stuck with the dirty plate.

Am I crazy to be irked that my teen Sims can't go to school at the downtown HS? I wouldn't have built it yet if I knew....

While the teens don't go to school at the downtown high school (they need to use the one in the main town unlocked after completing the Higher Education queued quest) they can at least do the Class Clown hobby.

It would have made a lot more sense for the downtown high school to be a university instead. They could have still allowed teens to do any hobbies there. There just wasn't a need for two high schools.